The present invention relates to cold-set adhesives for a cold corrugation system.
At present, as adhesives for corrugated board, starch adhesives comprising corn starch prepared by a Stein-Hall process are conventionally used. In the Stein-Hall process, a liquid mixture composed of a carrier solution and a main slurry is used as the liquid adhesive, but preparation of the liquid mixture is complicated. Further, in production of corrugated board with using such a starch adhesive, the liquid adhesive is applied to flute tips of the corrugating medium and then liners are adhered to the corrugating medium. In this case, solidification of the liquid adhesive is carried out by the so-called hot-corrugation system, which comprises compulsorily heating, by means of hot plates heated to from about 150.degree. C. to 160.degree. C.
An aqueous solution of polyvinyl alcohol has sometimes been used as the adhesive for corrugated board. However, this adhesive is expensive as compared with starch and it cannot be applied to high-speed corrugating machines because solidification takes a long period of time (because solidification is carried out by condensation with evaporation of water).
U.S. Pat. No. 4,094,718 has disclosed an adhesive comprising a powdered polyvinyl alcohol together with starch. This adhesive is prepared by dispersing a finely divided powder of polyvinyl alcohol in an aqueous solution of starch, which essentially is an improved variation of the Stein-Hall process. In this case, it is also necessary for solidification of the adhesive to compulsorily heat by means of hot plates. As described above, since much heat energy is consumed in Stein-Hall process today, economical problem is caused by energy consumption.
Further, from the viewpoint of quality of the corrugated board sheets, there is a problem in the case of solidifying by heating, in that water premeated into paper by application of the aqueous solution of starch is compulsorily removed by dring, which may cause warping because of uneven stretching of the paper, and such warping may created problems in a subsequent carton making process.
Recently, development of the so-called cold-set adhesives, by which adhesion is carried out without using hot plates, and development of cold-corrugation systems using such adhesives have been of great interest in view of abnormal increases in energy costs. For example, J. J. Becker, G. E. Lauterback, G. R. Hoffman and R. C. Mc Kee have done fundamental studies using a chemically modified starch solution prepared by adding sodium sulfite, NaOH, Na.sub.2 CO.sub.3 or NaHCO.sub.3 to pearl corn starch and additionally adding (NH.sub.4).sub.2 S.sub.2 O.sub.8 thereto, as has been described in Paper Board Packaging, Vol. 59, No. 6, p. 22 (1974). This adhesive is a so-called cold-set adhesive, which is used in liquid state at 88.degree. C. or more and gelatinizes at 60.degree. to 70.degree. C. However, the gelatinized adhesive is the so-called irreversible gel that the adhesive does not become liquid even if heated again to 90.degree. C. or more, when it is once gelatinized. A cold-corrugation system using the abovedescribed cold-set starch adhesive for corrugated board has been reported by Clude H. Sprague in Tappi, Vol. 62, No. 6, pp. 45-48 (1979). However, it has not been put to practical use up to the present time. There are problems with such a system, viz., that the initial adhesive property during the period from adhesion of the corrugating medium to liners to the process by a slitter is poor, and the liquid adhesive does not sufficiently premeate into the paper.
Furthermore, as a process of using synthetic resins, use of cross-linking water-soluble resins has been reported in Boxboard Containers, p. 26, May (1979). In this case, the adhesive becomes insoluble in water after adhesion and, consequently, recovery of waste paper becomes difficult.
On the other hand, there have been additional attempts at using hot-melt adhesives. In this case, the adhesives are expensive and recovery of waste paper is also difficult because the adhesives are insoluble in water.
Furthermore, a process using synthetic resin emulsions, such as a polyvinyl-acetate emulsion, has been proposed. This process also cannot be commercially utilized because this adhesive is more expensive than starch adhesives.
Recently, a cold-set adhesive comprising a polyvinyl alcohol as one component has been reported by Gerard de Knegt, of Switzerland, in Weekly Packing News, Nos. 737 and 738, Apr. 5 and Apr. 12, 1980. However, the composition of this adhesive is not fully described. It has been merely disclosed that when the solid content is 47% by weight and 33% by weight, the viscosity of the aqueous solution of the adhesive is 3,100 centipoises and 2,400 centipoises at the temperature of 25.degree. C., respectively, and the pH of the adhesive solution is from 6 to 7. Further, it has been reported that the adhesive is not used by a complete cold-set method, because adhesion is carried out with about six hot plates.